Nuclear Magnetic Resonance (NMR) spectroscopy is an instrumental analysis method enabling the determination of the chemical composition of samples. In this process, high-frequency (HF) pulses are radiated into the sample, which is located in a strong, homogeneous static magnetic field B0, and the electromagnetic reaction of the sample is measured.
The high-frequency pulses are radiated in and the reaction, which is also in the high-frequency range, is measured by a measurement probe. Generally, measurement probes are employed in superconducting magnet systems in which the homogeneous static magnetic field B0 is oriented along a “bore.”
The samples to be analyzed, comprising liquid, powdery, gelatinous substances, but also tissue samples, single crystals, glasses, or mixtures of different substances, may be loaded into cylindrical, in particular circular cylindrical, sample containers. This sample container is inserted into a recess in the measurement probe provided for receiving samples. HF transmitter/receiver assemblies in the form of HF coils or HF resonators are arranged around this recess in the measurement probe. The HF assemblies generate alternating electromagnetic fields in the recess, and thus in the inserted sample, when HF signals are applied to the HF assemblies. The alternating electromagnetic fields that are emitted by the nuclear spins in the sample as a reaction to the excitation are in turn received by elements of the HF transmitter/receiver assembly. The test volume of the magnetic resonance apparatus is defined by the intersection of the volume having a homogeneous static magnetic field B0, the sample volume, and the volume in which the alternating magnetic fields of the HF assembly are sufficiently strong.
Typically, an NMR experiment employs the same HF coil and/or the same HF resonator to excite the nuclear spins, i.e. as a transmitter coil and/or transmitter resonator, and to detect signals in a later phase of the NMR experiment, i.e. as a receiver coil and/or receiver resonator. In so doing, the corresponding HF paths of the NMR spectrometer are correspondingly switched from an HF transmitter to a preamplifier and HF receiver. Corresponding switching devices are known. According to the known principle of reciprocity, an HF resonator that is suitable for generating an alternating magnetic field in a test volume is also suitable for receiving alternating magnetic fields from the test volume. Likewise, an HF resonator that is suitable for generating an alternating electric field in a test volume is also suitable for receiving alternating electric fields from the test volume.
Owing to the extremely small signals to be measured during NMR experiments, the sensitivity in the detection phase often has especially high importance during the design of a coil assembly. An NMR measurement probe usually has one, and optionally two, preferred frequencies that are used for detection. Additional frequencies are utilized primarily for transmitting decoupling pulses and/or for polarization transfers.
HF resonators that are tuned to the NMR resonant frequency of the nuclear spin to be measured (for example, the nuclear species 1H, 13C, 15N, 2H, 19F) provide particularly high sensitivity when receiving signals. In general, it is favorable for the sensitivity for the HF resonator to be arranged as close as possible to the sample. If two different nuclear species that have different NMR resonant frequencies, e.g. 1H and 13C, or 1H and 19F, are both to be examined with high sensitivity, suitable geometric arrangement of the HF resonators may be required.
Hereafter, definitions will be provided for important terms as they are used in the present description and in the claims A “coil” is understood to be a continuous electrical conductor or a resonant structure having conductor portions and capacitive portions and components. A coil is arranged around a region in at least one winding.
A “flat coil” is understood to be a coil in which the conductors or conductor portions are arranged along a planar surface. In this case, one or more windings of the coil surround a region on the planar surface, which is also referred to herein as the “planar surface portion.” The planar surface portion may be, for example, in the shape of a rectangle or any polygon, optionally having rounded corners. When there are wide strip conductors or multiple windings, the current concentration on a strip conductor or the current concentration of adjacent conductor elements of multiple windings should be taken into account to determine a position or a dimension of the flat coil.
The term “axis of a coil” is understood to mean an axis through the region around which the windings of the coil are arranged and relative to which a winding direction of the coil is defined. A magnetic field generated by energizing the coil is oriented in the center of the coil approximately in the axial direction of the axis of the coil.
A “plate” is understood to be a planar component that has a greater extent in two directions in parallel with a plane than in a third direction that is perpendicular to the plane. An end face of a plate is understood to be a surface that defines the plate and extends substantially in parallel with the plane. An edge surface of a plate is understood to be a surface that defines the plate and is not an end face.
In one example of a prior art (e.g., German Patent reference DE 10118835 A1), an HF resonator assembly generates two independent alternating magnetic fields in a test volume. The test volume is delimited by a cylindrical sample tube, i.e., the sample tube extends along a central longitudinal axis. The HF resonator assembly comprises a first and a second resonator which are both arranged close to the test volume. This HF resonator assembly comprises a first pair of flat coils that are connected to form a first HF resonator. These flat coils are arranged on first coil support plates that are mutually parallel and in parallel with the longitudinal axis. These first coil support plates are positioned on opposing sides of the test volume. The HF resonator assembly further comprises a second pair of flat coils that are connected to form a second HF resonator and that are on second coil support plates that are mutually parallel and in parallel with the longitudinal axis. The second coil support plates are perpendicular to the first coil support plates and are likewise arranged on opposing sides of the test volume. Each end face of a coil support plate, i.e., a surface that is perpendicular to the coil axis of the particular flat coil, faces an edge surface of an adjacent coil support plate. The four coil support plates thus define a space that surrounds the test volume and is in the shape of a prism having a square or rectangular base. The coil support plates each project beyond this base in different directions. This example arranges the flat coils on the coil support plates such that a group of conductor structures extending in parallel with the longitudinal axis is arranged as close as possible to the test volume. This results in an assembly in which the flat coils associated with one HF resonator are arranged on one side of the longitudinal axis in each case when viewed perpendicularly to the coil support plates of said flat coils.
FIG. 5A is a cross section and FIG. 5B is the corresponding plan view of an HF resonator assembly according to the prior art disclosed in DE 10118835 A1. The flat coils 11 and 12, and likewise the flat coils 21 and 22, do not overlap in this case. This is because, according to the teaching of DE 10118835 A1, there must be longitudinal conductor elements on each flat coil that are arranged as close as possible to the test volume.